The prior art most closely related to that of the present invention is the technique for matching the impedance of radio transmission aerials to the impedance of the transmitter I(as described in "Electronic and Radio Engineering" by Frederick Terman, Library of Congress card number 55-6174 and "Radio Handbook" by William Orr, Library of Congress card number 40-33904). These prior art references refer to the need to match the impedance of each element of the circuit (transmitter, transmission line and aerial) to maximise the radio frequency (RF) energy radiation of the circuit at the required operating frequency. This impedance matching is carried out during initial set up of the system to operate at the fixed carrier frequency of the transmitter. Once the impedances are initially matched, no more tuning is carried out as the circuit does not substantially change its impedance during operation. The measurement of the system impedance is usually carried out by a directional coupler, which measures the reflected current returned from an ill-matched circuit. From this current the ratio of maximum to minimum voltage along the line (termed the `voltage standing wave ratio` or `VSWR`) can be calculated. A VSWR of unity indicates a perfectly matched system. The adjustment of the impedance of the system is usually carried out by varying the length of the aerial or the transmission line, or by adding lumped components such as inductors or capacitors to the circuit. The technique described is well understood by those skilled in the art of radio transmission of RF power for the transfer of information to a remote RF receiver. The present invention addresses the problem of measuring the impedance of a remotely mounted electrically resonant structure which is not designed for energy radiation. Such structures are usually used in oscillator circuits to control the frequency of oscillation. They are always mounted close to the controlling circuit because the impedance of connection wires between the structure and the electronics would cause the circuit to cease functioning. The impedance of these structures can vary with factors such as temperature, humidity and strain, and therefore they can be used as sensors. Previous applications have always mounted the controlling electronics close to the resonant structure, and provided separate connections (contact or non-contact) for power transfer to the control electronics and measurement of the signal. This technique leads to problems when the sensor is mounted in difficult situations, such as in a high temperature region, in an explosive environment (power to the sensor must be limited) or on a rotating shaft.
An object of this invention is to remove the need for the controlling electronics to be mounted near the structure, and to allow the impedance of the structure to be measured with very low signal power.